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BCD SPECIAL REPORT ON
HERITAGE RETROFIT
FIRST ANNUAL EDITION
35
INSULATION IN
TIMBER-FRAMED BUILDINGS
ROBERT DEMAUS
T
HERMALLY, TIMBER-FRAMED
walls generally perform badly
compared with other traditional
construction, and struggle to meet
modern expectations. This article
addresses the problems and risks
associated with retrofitting insulation
to upgrade their thermal performance.
It focusses on cases where the timbers
are exposed externally, which are usually
the most problematic, but also considers
timber frames which are concealed
behind cladding (either of the same
period or later).
There are circumstances in which
retrofitting insulation to a timber-framed
wall is acceptable and beneficial, but other
measures might prove more cost-effective
and less damaging. To determine the best
way forward, survey and analysis should be
carried out by an independent consultant
rather than by a materials supplier or
contractor. As well as comfort, cost-saving
and environmental gain, many other
factors must also be considered, including:
• The historic significance of the
building as a whole, as well as the
relative significance of individual
elements, and the degree to which
retrofitted insulation will alter it
• The condition of the building fabric
and the nature and extent of any
interventions (other than thermal
insulation) that might be necessary
• The causes of any existing degradation
and how these might best be remedied
• The current hygrothermal
performance of the timber-framed
walls and the building as a whole
• The ‘landscape value’ of the building
and the potential impact of any
change to its external appearance
• The performance of heating and hot
water systems and the cost benefit of
upgrading
• The condition and efficiency of
existing insulation, for example in roof
spaces and floors, and the cost benefit
of upgrading
• The potential for introducing
cost-effective and reversible new
elements such as secondary glazing
that do not involve significant
harm to historic fabric
• The building’s current use and the
occupants’ expectations.
The absolute and relative importance of
these and other factors will vary greatly,
not just between buildings, but between
areas of the same building.
The thermal performance of a
timber-framed wall is not only controlled
by its component materials. Condition,
orientation and exposure will have a far
greater effect on a 100mm thick timber-
framed wall than on a 225–350mm brick
wall. Moisture retention within the wall is
also critical to its thermal performance.
The large original verge overhangs of this historic timber-framed house protect the wall below, while an angled
‘pentice’ board above the ground floor window sheds water away from the wall below. (All photos: Robert Demaus)